Field of Invention
This invention relates to the field of Heating, Ventilation, and Air Conditioning (HVAC) and, in particular, to connecting a thermostat to an HVAC unit.
A Furnace Control Board is the control board that operates the furnace by controlling the furnace parts (such as the inducer motor, flame igniter, gas valve, blower, heating element), generally comes with the furnace, and is responsive to an external control such as a thermostat.
A Backchannel is a channel of communications that goes from the furnace control board or furnace control board adapter to the thermostat or thermostat adapter. This direction is opposite to the normal direction of signals which is from the thermostat to the furnace control board.
Polarity Splitting means that the positive and negative half-cycles of an AC signal may be used for different functions. Polarity Splitting will mean the same as Polarity Separation.
A Wireless Access Point (WAP) is a networking hardware device that allows wireless devices to connect to a wired network using Wi-Fi or related standards. The Wireless Access Point may connect to a router (via a wired network) as a standalone device, but it can also be an integral component of the router itself. The term Wireless Access Point will mean the same thing as Wireless Router.
The term UART means a Universal Asynchronous Receiver/Transmitter.
The terms House Wiring and House Cable mean the wires installed between the furnace and the thermostat regardless of the type of structure it is installed in. A House Wire is a wire in a House Cable.
The term Furnace Power means the low voltage used to operate the furnace control board. Furnace Power is typically 24 VAC.
The term “Line Sensor” means the same as “AC power phase detector.”
The term Relay means an electromechanical relay as well as a solid state relay.
Prior Art
There are several different type of furnaces and several different methods used to control them.
Some furnaces, especially electric heaters, require that the thermostat control a circuit operating at Mains power, typically 120 VAC.
There are Millivolt control systems where the furnace uses a thermopile in a pilot flame to produce power for the furnace to open the gas valve. This was one of the first control systems for gas heating appliances and they were installed on gravity type furnaces since the power from the thermopile was insufficient to run a blower in a forced air system. This method is still commonly used in gas hot water heaters because the heat from the pilot is not wasted. It simply adds to the heat for keeping the water hot. Also, a conventional water heater does not have a blower.
More commonly, thermostats control the furnace by closing a low-voltage two-wire circuit operating at the defacto standard of 24 VAC.
Before there were electronic thermostats the thermostats were mechanical devices that used a bimetallic strip made of two dissimilar metals that expand and contract at different rates as the temperature changes. An early patent that teaches the use of a bimetallic strip is U.S. Pat. No. 281,884 Electric Tele-Thermoscope issued Jul. 24, 1883 to Warren S. Johnson. (IDS Cite 1 Column 1, lines 16-34 and Column 2 lines 87-95) The two dissimilar metal strips are wound together in a spiral with the inside end fixed and the outside end controlling a beam lever. Under each contact point of the beam lever is a small cup of mercury. When the temperature is below the set point one end of the beam contacts its pool of mercury and the contact on the other end of the beam is lifted out of its mercury pool. When the temperature is above the set point the beam lever pivot reverses which pool of mercury is in contact with its respective beam lever contact.
This evolved so that instead of two open cups of mercury the mercury was in a sealed capsule with two electrodes. As before, the bimetallic strip is wound in a spiral with the inside fixed. However, instead of the open cups of mercury the mercury capsule is mounted at the free end of the spiral. The expansion and contraction of the bimetallic strip around the set point causes the mercury capsule to tilt one way or the other. One way causes the mercury to wet the electrodes and complete the circuit. The other way draws the mercury away from the electrodes opening the circuit. The mercury capsule is elongated to produce hysteresis. An example of a mercury switch thermostat is U.S. Pat. No. 1,822,605 Mercury switch thermostat issued Sep. 8, 1931 to Teeple. (IDS Cite 2) It should be appreciated that thermostats with mercury must be installed perfectly level or the set point will be wrong.
In other mechanical bimetallic strip thermostats the bimetallic strips directly make (or do not make) electrical contact with each other. Hysteresis is provided by a magnet. An example is U.S. Pat. No. 2,129,477 Adjustable metallic thermostat issued Sep. 6, 1938 to Parks. (IDS Cite 3)
These mechanical thermostats require only two wires to turn the furnace on.
Nowadays, although there are still mechanical thermostats most thermostats are electronic and need power to operate. An example of an early patent for an electronic thermostat is U.S. Pat. No. 3,942,718 Electronic thermostat issued Mar. 9, 1976 to Palmieri. {IDS Cite 4} This patent does not show where it gets its power from but since it uses discrete logic it is unlikely that the power came from batteries. When the patent application was filed in 1973 there were no microcontrollers, and microprocessors required considerable support circuitry, all of which required more power than was practical to get from batteries for long term use. Therefore, this thermostat would have required external power.
An example of an early patent for a User programmable thermostat is U.S. Pat. No. 4,442,972 Electrically controlled programmable digital thermostat and method for regulating the operation of multistage heating and cooling systems issued Apr. 17, 1984 Sahay, et al. {IDS Cite 5} This patent also required external power. See FIG. 5 element 35. It is the furnace transformer.
Since thermostats using the technology taught by both '718 and '972 would have required external power (such as furnace power) the sales of these thermostats would have generally been limited to new construction and to homeowners willing to install (or have installed) a new cable from the furnace to the thermostat.
Nowadays with the availability of very low power microcontrollers a thermostat may operate solely from its batteries. Some thermostats are designed so that the batteries may last for several years before they need to be replaced. Some thermostats may need its batteries replaced in as little as a year. If the batteries die the furnace will not work and the home's residents may wake up in the morning to a very cold house. If the residents are away at the time the failure of the furnace to operate may result in frozen and burst water pipes.
Some thermostats augment battery power by the process known as Power Stealing. In Power Stealing the thermostat operates on a small leakage current through the furnace controller input. This leakage current must be small enough so the furnace controller does not recognize it as a valid control input such as Call for Heat, Call for Cooling, or a manual fan control (Auto/Fan). An early patent that teaches Power Stealing is U.S. Pat. No. 4,211,362 Smoke detecting timer controlled thermostat issued Jul. 8, 1980 to Johnson. {IDS Cite 6}
Power Stealing generally uses only one furnace control input, such as Call for Heat. As a result, during a Call for Heat (which turns the furnace on) the thermostat contacts are closed and there is no power available to steal. There do not appear to be thermostats that steal power from more than one circuit. For example, in a system with both a furnace and an air conditioner they will not both be on at the same time. The reason for not stealing power from both is probably because furnaces and thermostats are made by different companies and there are few industry standards for the electrical characteristics of furnace controller inputs. As a result the thermostat designer cannot count on any particular furnace configuration or installation configuration and must make the thermostat compatible with as many different configurations as possible. This includes several generations of furnaces spanning several decades. Also, many thermostats are installed by homeowners who might not have a great deal of technical expertise.
There are thermostats which can operate on the power from the furnace control board (24 VAC). This requires that a connection be made to the 24 VAC Common terminal (commonly called the C Terminal) at the furnace controller. Operating from furnace power greatly extends the lifetime of the batteries which are then only backup batteries used during power outages to save the thermostat timer program and to keep the clock running. If the microcontroller in the thermostat contains non-volatile memory for storing the timer program then the batteries are needed only to keep the clock running.
There are thermostats, such as Wi-Fi thermostats, that require furnace power because of the power demands of Wi-Fi. An example is the Honeywell RTH6500WF. (IDS Cite 7)
However, that requires an additional wire between the furnace and the thermostat. Some cables already have the extra wire. It might even already be connected appropriately. If not, it is a simple matter to connect the end of the unused wire to the 24 VAC Common terminal on the furnace controller board to bring furnace power to the thermostat.
What if the cable from the furnace to the thermostat does not have the extra wire? Running an extra wire (or new cable) might be difficult and expensive. Depending on how the home is constructed it might not even be possible other than by running an exposed cable along the walls, a practice that most people find unsightly.
If an installed cable for a furnace and air conditioning system has four wires (24 VAC, Call for Heat, Call for Cooling, and Auto/Fan) the Auto/Fan wire can be repurposed as the 24 VAC Common. The thermostat will get furnace power but the Auto/Fan feature will not be available. The Auto/Fan feature allows the homeowner to run the fan (blower) to circulate the air when the furnace or air conditioning is not running and many homeowners consider this a valuable feature.
Likewise, if an installed cable for a furnace-only system has three wires (24 VAC, Call for Heat, and Auto/Fan) the Auto/Fan wire can be repurposed as the 24 VAC Common. Again, the thermostat will get furnace power but the Auto/Fan feature will not be available.
U.S. Pat. No. 3,815,668 Comfort control system and components thereof issued Jun. 11, 1974 to Carlson teaches using polarity separation of the AC power through the thermostat to control both heating and cooling control over two wires. (IDS Cite 8) This technique is also taught by U.S. Pat. No. 4,083,397 Heating-Cooling control system issued Apr. 11, 1978 to Kimpel et al. (IDS Cite 9) U.S. Pat. No. 5,452,762 Environmental control system using poled diodes to allow additional controlled devices in existing four wire systems issued Sep. 26, 1995 to Zillner uses this technique to allow a four-wire system to control a number of extra functions such a humidifier. (IDS Cite 10) None of these patents teach providing furnace power to the thermostat or providing a backchannel to allow the furnace to send an information signal to the thermostat. See Carlson FIG. 1 (IDS Cite 8), Kimpfel Figure (IDS Cite 9), and Zillner FIG. 2 (IDS Cite 10).
Controllers for modern gas furnaces perform a number of safety checks and shut down the furnace if a fault is detected. An example is if the temperature in the combustion chamber is too high, which may be the result of a failure of the blower or even just a very clogged air filter. Another example is if the gas valve is turned on but no flame is detected within a specified period of time. Such control boards will commonly flash a repeating code on an LED to indicate the nature of the fault. For example, many Ruud/Rheem furnace controllers use the following codes (IDS Cite 11):                2 Flashes—Ignition failure. This failure occurs when the control does not see proper ignition given the allotted attempts and ignition cycles.        3 Flashes—Pressure switch fails to open.        4 Flashes—Pressure switch fails to close.        5 Flashes—Neutral polarity failure.        6 Flashes—ECO failure.        
However, such furnace controllers do not bring an electrical error signal off-board. In order to see the LED on the furnace control board you usually have to remove the furnace panels. Even if the furnace has windows so you can see the LED without removing the panels you have to be physically at the furnace. Some furnaces are located in places like attics which are not easily accessible. Pointing either a wired or wireless camera at the LEDs on the furnace control board requires a window in the furnace cabinet to see the LEDs, a camera with an appropriate lens so the LEDs will be in focus, and room to mount the camera. It is an expensive solution, especially if an IP camera is used.
One furnace, the Rheem RGFG-Series, performs diagnostics and communicates the results to the thermostat through a wired serial link. However, this requires a proprietary thermostat. Rheem provides few technical details other than that it uses two AA batteries and it requires four house wires to connect to the furnace. {IDS Cite 12, PDF page 3}. It is currently very expensive. As of Mar. 9, 2016 it cost $357.70 (plus $4.49 shipping) on Amazon. {IDS Cite 12, PDF page 4} which is about 10 times the cost of a standard programmable thermostat.
U.S. Pat. No. 6,535,838 Furnace diagnostic system issued Mar. 18, 2003 to Abraham, et al. teaches extensive furnace diagnostics which may accessed by an infrared link to a remote handheld device or by an RS-232 interface to a modem. (IDS Cite 13) From the Abstract:                A furnace diagnostic system includes sensors that monitor various functions of the furnace. Data generated by such sensors may be stored for subsequent transfer or may be transferred in real time via an infra red link to a remote handheld device with which an analysis thereof is performed. The handheld device additionally allows the technician to control various furnace functions to facilitate the generation of relevant real time data. In order to further enhance the system's diagnostics capabilities, the communication may be established with a centralized computing facility which includes a data base containing data relating to an entire population of similar furnaces.        
Most thermostats use the defacto interface of closing contacts between the 24 VAC provided by the furnace control board and its inputs. Such functions commonly comprise: Call For Heat, Call For Cooling, and Auto/Fan. There is not even a standard for the furnace control board inputs which is why thermostats are required to provide isolated contact closures. An example of a standard thermostat is the Wright Rodgers P200. (IDS Cite 14)